Smith v Leech Brain & Co  2 QB 405 is a landmark case in English tort law that concerns the issue of causation in the context of damages for personal injury. The case established the eggshell skull or thin skull rule, which states that a defendant must take their victim as they find them, even if the victim has a pre-existing condition or susceptibility to injury that makes them more vulnerable than an average person.
In the case, the plaintiff was a worker at a factory who suffered a small burn on his lip while working with molten metal. The defendant, a manufacturing company, was found to be liable for the burn as it had failed to provide adequate safety measures to protect its workers from such injuries. However, the plaintiff had a pre-existing condition, a non-cancerous growth on his lip, which made him more susceptible to injury and also increased the risk of the injury developing into cancer. The plaintiff subsequently developed cancer and claimed damages for the full extent of his injury.
The defendant argued that they should only be liable for the burn, and not for the cancer, as the plaintiff's pre-existing condition made him more susceptible to developing cancer, and the burn only hastened the onset of the condition. The court rejected this argument and held that the defendant was liable for the full extent of the injury suffered by the plaintiff. The court established that a defendant must take their victim as they find them, even if the victim has a pre-existing condition or susceptibility to injury that makes them more vulnerable than an average person. The defendant is liable for the full extent of the injury caused, regardless of the victim's pre-existing condition. This principle is now known as the eggshell skull or thin skull rule.
You can learn more about this topic and other relevant case law with our Tort Law notes.